1. Field of the Invention
The present invention relates to means for supplying the high frequency energy and a high frequency electrodeless discharge lamp device using the same.
2. Related Art of the Invention
In view of the characteristics such as high efficiency and high color rendering, application of a high-intensity discharge lamp, in particular metal halide lamp, to a light source for the liquid crystal video-projector or the like has been advanced in recent years as a high output point light source substitutable for a halogen lamp. Besides, in view of characteristics such as high color rendering, development of its application also to a sports lighting corresponding to the high-definition TV broadcasting or the display lighting such as museum/gallery has been advanced.
Above all, a high frequency electrodeless discharge lamp has a merit, compared to electroded arc discharge lamp in that electromagnetic energy is easily connected to a filler, mercury can be saved from a filler for the discharge light emission and making the light emission highly efficient is desirable. Besides, because of there being no electrode inside a discharge space, no blackening of a bulb inner wall by the evaporation of electrodes takes place. Thus, the life of a lamp can be prolonged to a great extent. From these features, research and development of such electrodeless lamps has been extensively carried out as high-intensity discharge lamps of the next generation.
Hereinafter, a conventional high frequency electrodeless discharge lamp will be described referring to "Microwave Electrodeless Lamp" disclosed in Japanese Patent Laid-Open No. 59-86153.
That is, a conventional microwave electrodeless lamp is so arranged that the electrodeless lamp is provided in a microwave cavity resonator having an opening with the appendant mesh impenetrable to microwave and a microwave oscillator is linked therewith. Here, the maximal size of a discharge bulb in this electrodeless discharge lamp is smaller than the wavelength of a microwave used.
In such an arrangement, the microwave energy generated by a microwave oscillator is coupled to the discharge bulb through a slit on the wall of the above-mentioned microwave cavity resonator provided for the transmission of a microwave to excite an enclosed medium in the discharge bulb. In this manner, a radiant light generated from a microwave electrodeless discharge lamp is so arranged as to be taken out to the exterior of the microwave cavity resonator through a mesh provided at the above-mentioned microwave cavity resonator.
Like this, a cavity resonator has so far been generally used as high frequency energy supply means of an electrodeless discharge lamp device using a high frequency wave, especially microwave.
Incidentally, since generally in a discharge lamp device, the design of luminous intensity distribution can be more idealized with a smaller-sized light source, downsizing of a plasma arc as light source is widely required in the application field of illumination.
On the other hand, in electrodeless discharge lamps, the size of a plasma arc is determined by the inside diameter of the bulb. Accordingly, for the downsizing of a plasma arc, to make the inside diameter of the bulb smaller is needed.
With an arrangement which supplies energy to the bulb of an electrodeless discharge lamp by means of the cavity resonator of the above-mentioned conventional electrodeless discharge lamp, however, the coupling rate of microwave energy worsens and reflective waves increase if the size of a bulb is far smaller than that of the cavity resonator, thereby causing bad effects such as the worsening of the light emission efficiency and a drastic decrease in the starting function of lamp lighting. Consequently, the size of a bulb could not be reduced below the limit size determined by that of the cavity resonator.
Besides, with the above-mentioned arrangement of the above conventional electrodeless discharge lamp, a cavity resonator has been used as means for the supply of energy to an electrodeless discharge lamp. The size of a cavity resonator is determined by the wavelength of a high frequency wave applied. Besides, for the distinction from the band for a general information communication, the band of high frequency waves (ISM(Industrial, Scientific, Medical) band) available for industrial use is previously determined. For that reason, the size of a cavity resonator could not be reduced below the size determined by the wavelength limits of high frequency band available.
From these, there has been a problem that the size of a bulb cannot be reduced below that determined by the wavelength limits of high frequency band available.
For example, for a high frequency wave of 2.45 GHz (wavelength: 122 mm), commonly used ISM band, the size of a plasma arc capable of retaining a stable discharge is experimentally limited at and above about 15 mm.
On the other hand, in consideration of application to liquid crystal video projector, the size of a plasma arc below about 3 mm is determined from the convenience of optical design for a rise in the utilizing efficiency of emissions.
Accordingly, the high frequency electrodeless discharge lamp device using a cavity resonator had a problem of inappropriateness in applications where a point light source with a high luminosity is required. Thus, a high frequency energy supply means is intensively desired which can supply a high frequency electromagnetic resonance field concentratively into a smaller space than the cavity resonator.